1. Field of the Invention
The present invention relates to a structure of a rotor for a brushless motor provided with a sintered ring magnet.
2. Description of the Related Art
Various motors such as an induction motor, a commutator motor and the like have been conventionally used as a motor, however, a brushless motor having a high performance has been frequently used in recent years. In an inner rotor type brushless motor which is one of the brushless motor types, a magnet is arranged in an outer periphery of a rotor forming an inner rotor and is fixed to a rotor yoke by an adhesive agent. The magnet includes a so-called segment type in which a plurality of magnet pieces are arranged in a peripheral direction in correspondence to a number of poles of the brushless motor, and a ring magnet which is formed in a cylindrical shape and is magnetized in multi poles in a peripheral direction. However, in the segment type, an adhesive strength is weakened as a result of a temperature increase, and there is a risk that the magnet pieces are scattered in a centrifugal direction on the basis of the rotation of the rotor, and there is a problem that a reliability of the magnet pieces against the scattering is lowered if the number of the magnetic poles of the rotor is increased.
In order to prevent the magnet pieces from being scattered, there has been invented a structure in which a metal cylinder body is provided as a scatter preventing member in an outer periphery of the rotor, and a so-called magnet interior type structure in which holes are provided in a peripheral edge portion along an outer peripheral surface of the rotor yoke, and a magnet piece is fitted and inserted to each of the holes. However, in order to arrange the metal cylinder body, it is necessary to make a magnetic gap between the magnet piece and stator wide, so that there is a problem that the performance of the motor is reduced. Further, there is a problem that an eddy current is generated in the metal cylinder body and the motor efficiency is reduced. In the magnet interior type, because the rotor yoke exists in outer sides of the magnet pieces in the same manner, the magnetic gap becomes wide, and because the rotor yoke exists between the magnet pieces, all magnetic fluxes of the magnetic pieces do not make their way to the stator side, and the magnetic fluxes leak to the other magnet pieces in both sides via the rotor yoke, so that there is a problem that the performance of the motor is reduced.
Accordingly, the ring magnet is superior to the segment type magnet piece in that the magnet is prevented from being scattered. However, if the adhesive strength is weakened even in the case of employing the ring magnet, there is generated a problem that an idle running is generated with respect to the rotor yoke. On the contrary, there has been invented a structure in which a recess portion is formed in an end portion of an inner periphery of the ring magnet, a convex portion constituted by a key or a pin is provided in the rotor yoke, and the idle running is prevented by engaging the recess portion and the convex portion (for example, refer to Japanese Unexamined Patent Publication No. 2001-178042). Further, there has been invented a structure in which a protruding portion in an axial direction is formed in an inner periphery of the ring magnet, a recess portion in an axial direction is formed in an outer periphery of the rotor yoke, and the protruding portion and the recess portion are matched and fitted to each other, thereby preventing the idle running (for example, refer to Japanese Unexamined Patent Publication No. 2003-174745).
Because the compact structure and the high performance of the brushless motor have made progress, a neodymium type sintered ring magnet having a high magnetic flux density is frequently used in the rotor of the industrial high performance brushless motor. The sintered magnet is structured such that a magnetic field is formed in a magnet particle within a forming mold frame in which a core rod is arranged in a center of a cylindrical sleeve, however, an outer periphery and an inner periphery after forming is not formed in a complete round shape but formed in a polygonal shape or generates a strain, due to a thermal expansion and contraction caused by the sintering, and an orientation of the magnet particle. Accordingly, the outer peripheral surface and the inner peripheral surface of the ring magnet are ground so as to be finished in desired shape and size after being manufactured. However, in the case where the protruding portion, the recess portion and the like for fitting to the rotor yoke are formed in the inner periphery of the ring magnet as mentioned above, it is necessary to finish these protruding portion and the like in a desired dimensional precision, so that there is a problem that the grinding step becomes complicated, and a manufacturing cost is increased.
Further, the brushless motor having a high output has been used in an elevator, a power steering of a motor vehicle, and the like in place of the conventional hydraulic motor, however, in the brushless motor used for transporting a passenger or the like as mentioned above, it is necessary to prevent the idle running and the scattering from being generated as much as possible in view of a safety.